A significant detriment to the energy density of most batteries is posed by the battery's cathode. This is true for battery chemistries using, for example, lithium or nickel. Typically, oxidant is stored at the cathode at a molar charge capacity that is two to five times lower than that of the anode. Many fuel cells, on the other hand, use oxygen from the air as a source of oxidant. The existence of a continuous and virtually limitless oxidant source enables, in principle, high energy density. However, the use of hydrogen and organic fuels precludes high energy efficiencies due to problems with vapor pressure and balance-of-systems complexity, such as humidification and membrane issues. Metal-air electrochemical cells are able to combine the ultra-high anode capacity of batteries with the air-breathing cathode of fuel cells in order to achieve substantial energy densities that are relevant to modern energy demands.
However, metal-air electrochemical cells experience problems with vapor pressure when the metal-air electrochemical cell employs an aqueous electrolyte or any other electrolytic solution with an electrolyte salt dissolved in a solvent. As the water content of an aqueous electrolyte diminishes upon evaporation, reaction rates at the electrodes decrease, and the volume of soluble products lessens, lowering energy capacity. Additionally, the metal fuel corrodes as the protons in the aqueous solution are reduced to form hydrogen gas. These problems may lead to eventual failure of the cell.
These problems have been addressed by using a low or room temperature ionic liquid rather than an aqueous electrolyte in a metal-air electrochemical cell, as described in U.S. Provisional Application Ser. Nos. 61/177,072, filed May 11, 2009, and 61/267,240, filed Dec. 7, 2009, both now converted as U.S. patent application Ser. No. 12/776,962, filed May 10, 2010. The use of a low or room temperature ionic liquid in the cell essentially eliminates the problems associated with evaporation of solvent from an electrolytic solution. Room temperature ionic liquids have extremely low vapor pressures (some have vapor pressures that are essentially immeasurable under standard conditions) and thus experience little or no evaporation. Therefore, cells using low or room temperature ionic liquids as their ionically conductive media need not incorporate excessive volumes of solution in order to compensate for evaporation over time. Relatively small amounts of ionic liquid are sufficient to support the electrochemical reactions needed for cell operation, thereby reducing cell weight and volume and increasing power to volume/weight ratios. Also, other problems associated with solvents, such as hydrogen evolution in an aqueous solution, may be avoided. This inventive development is not conceded to be prior art and merely is described for contextual purposes to facilitate an understanding of the further development described herein.